With increasing interest in digital image processing, various image processing technologies, apparatus or techniques are suggested. The image processing is utilized in various industries to improve the human visual perception.
The image processing includes feature extraction, image enhancement, image restoration, image reconstruction, image analysis, image recognition or image compression. The image processing employs techniques appropriate for its objects mentioned above.
In the case that an image of an object is taken under limited conditions such as a physical limitation of an image pickup device, characteristic of the object or various external environments where the object is located, the effects of the limited conditions are reflected on the resultant image.
A method for correcting a luminous intensity of an area of an image is used to address the above-mentioned image enhancement problem. To reflect an actual appearance of an object to increase human perception to the resultant image of the object, various methods are suggested, for example histogram stretching, gamma correction, algebra conversion or compulsory adjustment of brightness coefficient value.
Among the conventional methods, stretching in image processing is represented by the following Equation 1.
                              g          ⁡                      (                          x              ,              y                        )                          =                              (                                                            f                  ⁡                                      (                                          x                      ,                      y                                        )                                                  -                                  g                  min                                                                              g                  max                                -                                  g                  min                                                      )                    ×          255                                    Equation        ⁢                                  ⁢        1            
In the Equation 1, g(x,y) is a stretched image information, f(x,y) is an original image information, gmax is a maximum value of a pixel, and gmin is a minimum value of a pixel. The stretching is a technique for processing an image by the above-mentioned equation, and adjusts to stretch a pixel value distribution of an image having a low contrast more widely.
And, the gamma correction or algebra conversion uses a specific function such as an exponential function or a log function, in which an original image information is an independent variable and a function value of the function is a dependent variable, and the function value corresponds to the resultant image.
However, the above-mentioned techniques do not take into account the characteristic of the entire image based on human perception to brightness of the image, and uses a method for uniformly changing brightness information of the image by conversion to increase the luminous intensity of a low luminous intensity area.
In this case, a dark area may be expressed as a bright area, however the brightness and color of the entire image is changed, or the brightness and the color of the original image is considerably influenced by distortion of image information such as color twist of a specific area.